VOLCANIC
DOME
CAP ROCK .
High heat flow from regions of
partly melted rock in the earth's
upper mantle, about 30 miles
deep, warms water that seeps
underground. The water then
migrates back toward the surface
along fractures or returns by well
tapping. At the surface it vents as
boiling water and steam, which
can be harnessed to spin turbines.
Pockets of magma, or molten rock,
transfer intense heat to surrounding
rocks and water that may be
trapped underground. If enough
water collects in porous rock
"reservoirs," it can be recovered by
wells. Attempts to tap the heat of
magma directly have been blocked
by temperatures as high as 2200°
Fahrenheit (12000 Celsius).
Tapping heat from
0i-
earth'sdepths
FOUND throughout the world in surprising
abundance, geothermal systems vary in
makeup but share a common heat
Hawaii
source-the natural radioactivity that
exists in all rocks. With oil and coal rising
in cost and no longer considered inexhaustible,
earth's heat offers potential power that intensive
development may harness.
A government survey estimates that known
geothermal resources in the United States
could produce 140,000 megawatts over a life
expectancy of 30 years, equivalent to the output
of 140 nuclear plants.
The Geysers
Imperial
Valley
CO
ELSALVADOR
^/E^LV p
Q
federal lands have
been leased for
explorationsince
1974, mainly in the
West and on the
Gulf Coast.
*
Geothermal power plants
o
Geothermal power sites under exploration
A Major nonelectric geothermal sites
SPlate boundaries and fracture zones
'4,